1. Field of the Invention
The present invention generally relates to a sound image localization apparatus, a stereophonic sound image enhancement apparatus, and a sound image control system suitably used in various acoustic devices, for instance, electronic musical instruments, game machines, and sound mixers. More specifically, the present invention is directed to a sound image localization apparatus capable of realizing sound image localization by a loudspeaker by employing a simple analog circuit, to a stereophonic sound image enhancement apparatus capable of enhancing a sound image in response to a stereophonic sound signal in a two-channel loudspeaker reproduction, and to a sound image control apparatus for localizing a sound image to an arbitrary position in a three-dimensional space in response to a monophonic sound signal.
2. Description of the Related Art
Conventionally, such a technical idea is known in the field that 2-channel stereophonic signals are produced, and these stereophonic signals are supplied to right/left loudspeakers so as to simultaneously produce stereophonic sounds, so that sound images may be localized. In accordance with this sound image localization technique, the sound images are localized by changing the balance in the right/left sound volume, so that the sound images could be localized only between the right/left loudspeakers.
To the contrary, very recently, several techniques have been developed by which sound images can be localized outside the right/left loudspeakers. That is, in the first prior art to reproduce the sounds by way of the two right/left loudspeakers, the sound having the reverse phase with respect to the phase of the right-channel sound is mixed with the left-channel sound, and also the sound having the reverse phase with respect to the phase of the left-channel sound is mixed with the right-channel sound. As a result, the sound image may be localized outside the left/right loudspeakers. This sort of conventional technique is disclosed in, for instance, WO94/16538 (PCT/US93/12688) entitled "SOUND IMAGE MANIPULATION APPARATUS AND METHOD FOR SOUND IMAGE ENHANCEMENT."
Concretely speaking, in this first conventional technique, the difference signal between the left-channel left input signal and the right-channel right input signal is produced. This difference signal is supplied to the band-pass filter while the amplitude of this difference signal is properly controlled. Then, the difference signal derived from the band-pass filter is added to a one-channel input signal, so that the output signal for this channel is produced. Similarly, the difference signal derived from the band-pass filter is subtracted from the other-channel input signal, so that the output signal for this channel is produced. The respect output signals are supplied to the right/left loudspeakers. In accordance with this first conventional technique, since the sound image can be localized outside the right/left loudspeakers, the sound stage can be greatly enhanced.
Also, as the second conventional technique, the sound image localization technique called as "Schroeder" system is widely known in the field. In this Schroeder system, the sounds which are produced from the left loudspeaker and then reach the right ear of the audience, and also the sounds which are produced from the right loudspeaker and then reach the left ear (will be referred to as "crosstalk sounds" hereinafter) are canceled, so that the sound listening conditions through the headphone are established. As a consequence, the sound images can be localized not only between the right/left loudspeakers, but also arbitrary positions, for example, a position on one side of the audience.
Furthermore, as the third conventional technique, such a technique is known that since the head related transfer function (head related acoustic transfer function) is added and the crosstalk canceling process is performed by way of the convolution calculation, the sound image can be localized at an arbitrary position (for instance, see "As to RSS" by Roland K. K. JAPAN, Japanese Acoustic Society volume 48, No. 9).
However, the above-described first conventional technique has the following problem. That is, when the sounds are heard at a position apart from the loudspeakers, the audience hears the sound images which are localized outside the right/left loudspeakers. To the contrary, when the sounds are heard at a position close to the loudspeakers, the audience cannot cleary discern where the sound images are localized.
Also, the first conventional technique has another problem. That is, when the mixing ratio of the other-channel sound having the reverse phase to one-channel sound is increased by controlling the magnitude of the difference signal in order to increase the sound stage enhancement effect, the sound quality deteriorates. This quality deterioration is caused because the comb filter characteristic is formed by the monophonic components of the input signal. This sound quality deterioration will appear as such a phenomenon that the audience hears the sounds from which the low frequency components are mainly cut off. When the sound quality deterioration becomes extreme, it becomes difficult to reproduce an input source.
Also, the second conventional technique has a problem that the audience hear the sound image which is close by, resulting in unnatural sounds. In addition, if the above-described crosstalk canceling theory by the Schroeder system is strictly applied to constitute the sound image localization apparatus by employing the analog circuit, then a very large amount of hardware is necessarily required. On the other hand, if the sound image localization apparatus is constituted by employing the software of the digital processor (DSP) or the CPU, execution is very different. As a consequence, conventionally, the sound image localization apparatus with employment of the Schroeder system could be limitedly applied only to the high-grade electronic musical instruments and the high-grade acoustic appliances.
Also, the above-explained third conventional technique owns another problem. That is, when the head related transfer function is added and the crosstalk canceling process operation is carried out by way of the convolution calculation, a large number of convolution stages is required, so that the hardware scale is increased. Conversely, if a total number of the convolution stages is decreased, then a different problem occurs. That is, the low frequency components of the signals which are processed by the head related transfer function are reduced, and also the crosstalk sounds canceled by the crosstalk canceling process are reduced.